# Bootroms

## Attribution

The bootrom source code here has been adapted from the [SameBoy project](https://github.com/LIJI32/SameBoy/)
The MIT license is included in these files.

## Compilation

Bootrom compilation follows the same approach as [SameBoy](https://github.com/LIJI32/SameBoy/#compilation).

The following tools and libraries are required to build bootroms:

* clang or GCC
* make
* [rgbds](https://github.com/gbdev/rgbds/releases/)
* [SRecord](https://srecord.sourceforge.net/)

The Makefile contains to following targets:

* `default` compiles `dmg_boot.bin`, `cgb_boot.bin` and `sgb_boot.bin` and concatenates them into `cgb_boot.mif`
* `bootroms` compiles all `default` binaries as well as `mgb_boot.bin`, `cgb0_boot.bin` and `sgb2_boot.bin`
* `checksum` compiles a program to calculate simple checksums of binary files
* `all` all of the above
* `clean` delete all bootroms and object files, as well as `cgb_boot.mif`

## Checksum

A tool is provided to calculate the checksum of a binary file. These checksums are used in the core to verify if the enhanced boot features are available.

**Note: If these bootroms are edited and recompiled in future, the checksums should be recalculated and updated in [Gameboy.sv](../Gameboy.sv)**

## Enhancements

For convenience, two enhancements have been added to the bootroms which enables the on-screen display to change the default boot behaviour without the user needing to load a new bootrom.  
Some of these enhancements extend to alternative roms in the [binary folder](./bin)

### Fast boot

Fast boot skips the boot animation for the following bootroms:

* `cgb_boot.bin` (built-in)
* `cgb0_boot.bin`
* `dmg_boot.bin` (built-in)

### AGB emulation

The Game Boy Advance had a slightly different CGB bootrom, which enabled special features in a small number of games. AGB emulation is available for:

* `cgb_boot.bin` (built-in)
* `cgb0_boot.bin`
* The original CGB bootrom (not provided)